PHP 4 and 5 at same time?

I am testing out the latest edge with new easyapache and there are options to run both php 4 and 5. I was wondering if anyone knows how this works? Is it just a separate AddHandler for a different filename extension which will run php 5 as opposed to php 4 (like phpinfo.php5)?

Running both for development sounds like a neat idea to me. But I am wondering if this might be a feature which could potentially break things.